I know isn't exactly answering the question, but I went from Arcam A32, to bi-amping with a P35 (big improvement in soundstaging, bass clarity and dynamics), to using a Creek passive pre-amp with the A32 (in power only mode) and P35 bi-amping (massive improvement in detail and dynamics), to swapping the A32 for a P38 (a tad more bass resolution but in all honesty not much).
So if nothing else I think that demonstrates that
a)Bi-amping really works
and
b) A good (but not necessarily expensive) pre-amp will outperform the pre section of even a well respected mid-range integrated - slotting that creek pre amp into my system at the cost of £120 all-in from ebay was an unbelievably cost effective upgrade - I'd have been happy with the improvement if I'd paid £1k!
Having said all that, as I have got used to each improvement I still come back to this feeling that I'm lacking something in terms of impact and immediacy so I'm hankering after trying something from Naim maybe...